I pay £15 a month on O2 Pay As You Go and that gives you 3GB I will find out more info.
Quote from a website announcing the tariffs for Mobile services.
O2 has announced that it will be offering three tariffs. For £2 a day, iPad owners can access up to 500MB, without having to sign any contract. It also has two monthly payment options: for £10 a month iPad users get 1GB of usage and for £15 they get 3GB. Customers can cancel their contracts – with 24 hours' notice – at the end of the month.
I would recommend the £15 but depends how much you want to pay and also how often you will use the Internet
Hope that helps